Double beam hydrocarbon gas detector

ABSTRACT

Hydrocarbon gas in a sample is measured by a double beam detector responding to the transmission of absorption properties of the gas in response to applied radiation from a laser source. Monochromatic light from the laser source is split into two radiation beams, a sample path beam and a reference path beam, and directed to a radiation detector which sequentially responds to the reference energy and the sample energy. A difference in the absorption by the sample path energy with respect to the reference path energy produces two, different level, output signals from the detector. The sample energy signal and reference energy signal are respectively directed to a sample channel circuit and a reference channel circuit. Outputs from the individual circuits are amplified logarithmically and applied to inputs of a differential amplifier wherein the ratio of the sample signal to the reference signal produces a voltage for actuating a meter indicator. For a hydrocarbon gas content in a sample in excess of a predetermined limit, an alarm circuit actuates to give an audible warning of an excessive gas content.

United States Patent [1 1 Dennis [451 Sept. 25, 1973 [2]] Appl. No.:269,317

[52] U.S. Cl ..250/565; 356/205; 250/209, 340/237 R [51] Int. Cl. G0ln21/26 [58] Field of Search 250/218, 43.5 R, 250/209; 356/179-181, 201,204-208, 209; 23/232 R, 232 E, 254 E; 340/237 R [57] ABSTRACTHydrocarbon gas in a sample is measured by a double beam detectorresponding to the transmission of absorption properties of the gas inresponse to applied radiation from a laser source. Monochromatic lightfrom the laser source is split into two radiation beams, a sample pathbeam and a reference path beam, and directed to a radiation detectorwhich sequentially responds to the reference energy and the sampleenergy. A difference in the absorption by the sample path energy withrespect to the reference path energy produces two, different level,output signals from the detector. The sample energy signal and referenceenergy signal are respectively directed to a sample channel circuit anda reference channel circuit. Outputs from the individual [56] ReferencesCited If (H n d F dt circui s are amp I ie ogari mica y an app ie 0 in-UNITED STATES PATENTS puts of a differential amplifier wherein the ratioof the 3,447,370 6/1969 Tanzman 250/218 X Sample Signal to therefel-ence signal produces a voltage {2 7 et for actuating a meterindicator. For a hydrocarbon gas Son content in a sample in excess of apredetermined limit, 3,677,652 6/1971 Little 250/218 X l dbl f 3,690,7729 1972 End] 356/179 3 arm 9 actuates 1 e wammg 9 an excessive gascontent.

Primary ExaminerWalter Stolwein Attorney D. Carl Richards et a1. 21Clalms 5 Drawmg Figures l 34 0 ,..-ww-l-nr,./L, ,TLUJ

- 28 g i AIR 3O SAMPLE 32 EX HAUST OUTLET PATENTED SEPZS ma SHEET 3 OF 3mom 1 DOUBLE BEAM HYDROCARBON GAS DETECTOR This invention relates to thedetection of low level concentrations of hydrocarbon gas and moreparticularly to a dual beam hydrocarbon gas detector responsive toconcentrations of hydrocarbon vapors which may constitute a toxic,combustible, or explosive hazard.

Heretofore, gas detectors of the type generally in commercial use haveemployed Nerhst glowers, or glow bars, as an infrared source of systemenergy. Such conventional incoherent sources, to operate properly, mustbe of a fragile construction and therefore have a relatively shortuseful life. More importantly, detectors of this type have limitedsensitivity which may be increased only at the expense ofincreased'complexity and bulk of the detector system. Anotherdisadvantage of conventional detectors that employ incoherent energysources is that the transmitted beam power varies a direct function ofthe aperture size of the transmitting optics. As a result, the power,and therefore the signalto-noise ratio, ofthe beam received by receivingoptics and in turn a detector system is relatively low and a limitingfactor on system sensitivity. Circuits responsive to systems employingincoherent radiation detectors thus lack desired accuracy due tovariations of the output signal source power and the noise-to-signal'ratio.

A feature of the present invention is to provide a hydrocarbon gasdetector employing a coherent light source in the form of a laserproducing energy in the wavelength region of 3.39 microns. Energyprovided by the coherent energy source is split along a sample path anda reference path and directed to a radiation detector. Because of thecoherent nature of the laser beam, maximum power from the source to thedetector is transmitted thereby providing a desirable signal-tonoiseratio output from the detector. A sample beam signal and a referencebeam signal from the radiation detector is applied to a differentialmeasuring system that provides automatic compensation for power sourcevariation to provide consistently accurate meter readings of gasconcentration.

In the simplest form of a double beam system as employed in the presentinvention, radiationfrom a laser source is split into two beams so as topass along two separate paths of different length which include a samplepath through a sample chamber and a reference path directly to aradiation detector wherein the intensity of the two beams provide outputsignals for comparison in a responsive circuit. The radiation sensitivedevice, such as a photo multiplier tube, senses alternately radiationpulses from the sample beam and reference beam to generate separatesignals for application to amplifier circuitry that produces a compositeseries signal of the alternating reference and sample pulses, theamplitude of which corresponds to the intensity of the light transmittedthrough the sample and reference path. The detector and the circuitresponsive thereto are synchronized with the operation of a beam switchor chopper to provide the required degree of signal separation such thatan accurate meter reading may be produced that is proportional to theconcentration of hydrocarbon gas in the chamber of the sample beam.

In accordance with the present invention, a hydrocarbon gas detectorusing a monochromatic light source emitting radiation in a regionabsorbent of hydrocarbon molecules includes means for splitting thelight source beam into a sample path and a reference path both directedto a detector having an output related to the energy in the sample pathand reference path. Responsive to the detector output related to thesample path signal and reference signal is a sample path circuitresponding to the output signal of the detector related to the samplepath energy and a reference path circuit responding to the output signalof the detector related to the reference path energy. A logarithmicamplifier connected to the output of the sample channel circuit and thereference channel circuit generates two outputs; one varying as alogarithm ofthe sample channel output and the second varying as thelogarithm of the reference channeloutput. Responsive to the two outputsignals is a'differential amplifier having an output varying as thelogarithm of the ratio of the sample channel output over the referencechannel output. This output signal from the differential amplifier isapplied to an output circuit that generates at a meter a measure ofhydrocarbon gas in'a sample chamber.

More specifically, the hydrocarbon gas detector of the previousparagraph includes timing circuitry for establishing a timingintervalfor connecting'the sample channel circuit to the detector outputto respond only to the sample path energy and for alternately connectingthe reference path circuit to the detector to respond only to thereference path energy.

A more complete understanding of the invention and its advantages willbe apparent from the specification and claims and from the accompanyingdrawings illustrative of the invention.

Referring to the drawings:

FIG. 1 is a schematic of the hydrocarbon gas detector with a laserenergy source providing an energy beam chopped into a reference beam anda sample beam directed to a radiation detector;

FIG. 2 is a block diagram of a sample channel circuit and a referencechannel circuit responsive to the output of a radiation detector forproducing a meter indication of hydrocarbon gas concentration;

FIG. 3 is an end view of the chopper disc for splitting the laserenergybeam of the system of FIG. 1 into a reference beam and a sample beam;

FIG. 4 is a detailed circuit schematic of the sample and referencechannel circuits and the logarithmic and differential amplifiersresponsive to the outputs of both channels; and

FIG. 5 is a logic diagram for establishing timing signal to sequence theoperation of the circuit -of FIG. 4.

Referring to the FIGURES, there is shown in FIG. 1 a sample chamber 10with a suction fan 12 mounted at an open end thereof for drawing an airsample in the sample chamber for evaluation of hydrocarbon gas content.A sample drawn into the chamber 10 exhausts through an outlet 14 toatmosphere.

To evaluate a sample within the chamber 10 for hydrocarbon gas content,a laser 16 produces a monochromatic energy beam 18 to a chopper wheel 20driven by a motor 22. Rotation of the chopper wheel 20 alternatelypasses the beam 18 to a mirror 24 and a mirror 26. An energy beamincident on the mirror 24 is reflected therefrom to a mirror 28 forfurther reflection to a mirror 30 and subsequently to a radiationdetector 32. A light path established from the mirrors 24, 28 and 30 tothe detector 32 constitutes a sample path beam through the samplechamber 10. With the chopper wheel 20 in a position to reflect the beam18 to the mirror 26, an energy beam is further reflected to theradiation detector 32. This light path from the chopper wheel 20 to themirror 26 and the radiation detector 32 is a reference path beam andpasses through the sample chamber for a relatively short distance ascompared with the sample path beam reflected from the mirror 30.Positioned immediately in front of the radiation detector 32 is ablocking filter 34 for preventing various signals from impinging on thedetector 32, and further protects the detector from the corrosive andabrasive constituents of the gas sample drawn through the sample chamber10.

Although the laser 16 may be any energy source providing radiation in abandwidth absorbent of hydrocarbon gas, the helium-neon laser emissionat 3.39 microns is particularly well suited because it is near thefundamental vibration frequency of the coupling between the hydrogen andcarbon atoms. The absorption of 3.39 micron wavelength light byhydrocarbon gases is expressed mathematically by the Lambert-Beers Lawas follows:

where P/P, is the ratio of transmitted power in the sample beam and thereference beam through a gas sample in the chamber 10,

a is a constant peculiar to the particular gas,

x is the light path length of the sample path, and

p is the partial pressure of the gas within the chamber 10. FIG. 1illustrates a system wherein signals from the radiation detector 32 arecoupled to circuitry for evaluating equation (1) to give a measure ofhydrocarbon gas in an air sample drawn into the sample chamber 10.

Referring to FIG. 2, there is shown a block diagram ofa system forproviding a measure of hydrocarbon gas in the sample chamber 10 inaccordance with equation (1 above. Energy from the laser 16 after beingchopped into a sample path and a reference path by means of the chopperwheel impinges on the radiation detector 32. As an example of aradiation detector, it may be an indium-arsenide or lead seleniumphotodiode; such photodiodes have been found to be reasonably efficientat detecting 3.39 microns wavelength light energy. Output signals fromthe detector 32 are applied to a clamper circuit 36 which functions tobias the output of the detector to zero during dark current conditions,as will be explained. Operation in this mode produces less dark currentdistortion and bias shift. An output of the clamper 36 is applied to anamplifier 38 for amplification of the clamper signal to a level foroperating peak detector circuits. A gated peak detector sample channelcircuit 40 and a gated peak detector reference channel circuit 42 areboth connected to the output of the amplifier 38.

Upon rotation of the chopper wheel 20 by the motor 22, energy from thelaser 16 is sequentially split into a reference beam and a sample beam,as explained. During the time interval of the existence of the samplepath beam, the peak detector 40 is gated on by the output of a timingand logic circuit 44. Gating on the detector 40 applies the output ofthe amplifier 38 to the detector circuitry wherein it is stored forsubsequent processing. Rotation of the chopper wheel 20 to establish areference path causes the logic circuit 44 to turn off the peak detector40 and gate on the peak detector 42. During this time frame, an outputfrom the amplifier 38 is applied to the peak detector 42 wherein it isstored for subsequent processing.

To establish the timing sequence for gating on the peak detectors 40 and42, a photodetector 46 is positioned to respond to light from a source48 during rotation of the chopper wheel 20. As shown in FIG. 3, thechopper wheel constitutes a circular disc having removed sections whichallows the transmission of energy from the laser 16 to the mirror 24 toestablish the sample path and also permits transmission of light fromthe source 48 to the detector 46. When the motor 22 rotates the disc 20such that a mirror 50 is in the path of the beam l8, the reference pathis established by reflection from the disc mirror 50 to the mirror 26and subsequently to the radiation detector 32. During this timeinterval, light from the source 48 is blocked from the photodetector 46.An output from the photodetector 46 steps the timing and logic circuit44 to gate on the peak detectors 40 and 42 at the appropriate time toaccept signals from the amplifier 38.

In order to conserve power, the laser 16 is turned on only a smallportion of the available time; that is, it is pulsed approximately fivetimes per second, in a typical application, resulting in a duty cycle of12 1% percent. This is accomplished by a high voltage gate signal online 52 from the timing and logic circuit 44 as applied to the highvoltage gate terminal 54.

Coupled to the peak detector circuit 40 is a gain control 56 foradjusting the output of the detector to match the output of the detector42 under conditions of no hydrocarbon gas in the sample chamber 10. Alsocoupled to the output of the reference peak detector 42 is an alarmcircuit 70 driving an audible alarm 72.

An output from the peak detector 40 and the peak detector 42 is appliedto inputs of a logarithmic amplifier 58 which accepts the inputs,converts each into a voltage proportional to the logarithm of the inputand provides two output signals varying in accordance with the detectoroutputs. Connected to the logarithmic amplifier 58 is a differentialamplifier 60 that accepts each of the outputs of the amplifier 58 onseparate input channels. The differential amplifier 60 produces oneoutput signal that is proportional to the logarithm of the ratio of thesample path energy to the reference p'ath energy. This signal is appliedto a scaling circuit 62 that provides various instrument sensitivitiesby means of a range switch 64. An output of the scaling circuit 62 isapplied to a meter amplifier 66 for coupling to a meter movement 68.

In operation of the circuit of FIG. 2, an output of the detector 32 is aseries of pulses of two types. The first is a series of pulses whoseamplitude varies in proportion to the light intensity of the referencepath as established by the mirror 26 and the second type is a series ofpulses whose amplitude varies in proportion to the light intensity ofthe sample path as reflected from the mirror 30 through the samplechamber 10. The sample path traverses the length of the test chamber11th twice and is thus subjected to a greater absorption by hydrocarbongases present in the sample chamber than the reference beam whichfollows a relatively short path from the mirror 26 through the chamber1% to the detector 32.

Each of the pulse trains is sequentially generated by operation of thechopper wheel 20 and the timing logic 44 and is applied to the clamper38 and amplified in the amplifier 38. The pulse train of the sample pathis applied to the peak detector 40 which is gated on only when thesample pulses are present. Within the peak detector 40 a capacitor ischarged to the peak value of the sample pulses. In a similar manner, theseries of pulses from the reference path are applied to the peakdetector 42 wherein a capacitor is charged to the peak value of thereference pulses. The output of each peak detector 40 and 42 is appliedto the logarithmic amplifier 58 having outputs proportional to thelogarithm of the inputs. The difference between the two inputs of thelogarithmic amplifier 58 is a voltage representing the difference oflogarithms (equal to the logarithm of the ratio of PIP, of equation 1)of the two inputs. This difference signal is generated by applying thetwo outputs of the logarithmic amplifier 58 to the difference amplifier60.

If the Lambert-Beers Law as expressed in equation (1 is implementeddirectly from the peak detectors 40 and 42, an output voltage would begenerated which varies exponentially with gas concentration in thesample chamber 10. By taking the logarithm of the sample path signal andthe reference path signal, the output voltage of the differentialamplifier 60 varies linearly with gas concentration. This linearlyvarying voltage is applied to the scaling amplifier 62 for operating ameter movement 68 after further amplification in the meter amplifier 66.

As the concentrations of hydrocarbon within the sample chamberincreases, the amplitude of the series of pulses from the detector 32representing the sample path output decreases. Thus, the amplitude ofthe series of sample path pulses varies inversely with hydrocarbonconcentrations in the chamber 10. At some level of concentration ofhydrocarbon within the sample chamber 10, the output of the detector 32will be driven to some minimum value when responding to the sample pathenergy. This renders ineffective the sample peak detector 40. At thesehigh concentrations of hydrocarbons within the sample chamber 10, theintensity of energy in the reference'path begins to diminish due toabsorption by hydrocabon molecules. This absorption by hydrocarbonmolecules of the reference path energy causes a decrease in theamplitude of the series of pulses from the detector 32 for the referencepath. The'output of the reference peak detector 42 begins to decrease inproportion to the increased concentration of hydrocarbons within thesample chamber 10. Such a high concentration of hydrocarbon gas withinthe sample chamber 10 is an indication of a possibly explosive mixture.As the concentration reaches a preestablished upper level, the output ofthe reference peak detector 42 drops below a set point that triggers thealarm circuit 70. Triggering the alarm circuit 70 turns on the audiblealarm 72 giving an audible signal that a dangerous concentration ofhydrocarbon gas has been drawn into the sample chamber 10.

Referring to FIG. 4, there is shown a preferred circuit for implementingthe Lambert-Beer's Law to produce a linearly varying voltage to a metermovement 68 in response to the output of the detector 32. Detector 32comprises a lead selenium photodiode 84 coupled to the output of afilter network including resistors 74, 76 and 78 and capacitors 80, 82and 84. A coupling capacitor 86 in series with a resistor 88 appliessignals from the diode 84 to one input of an operational amplifier 90having a second input connected to ground through a resistor 92. Coupledacross the inputs of the amplifier is a diode 94 and connected to theoutput are two diodes 96, 98. A feedback resistor interconnects betweenthe cathode of the diode 96 and one input of the amplifier 90.

Amplifier 90 and circuitry associated therewith comprise the clampercircuit 36 that has an output at the diode 96 connected to one input ofan operational amplifier 102. A feedback circuit including a resistor104 in parallel with a capacitor 106 couples to a second input of theamplifier 102. A variable resistor also connects to the second input ofthe amplifier 102 and to ground.

Amplifier 102 and circuitry associated therewith comprise the amplifier38 having an output connected to an operational amplifier 110 and anoperational amplifier 112; the latter through resistors 114 and 116.Amplifier 110 has an output tied to a four-position switch 118 and avariable resistor 120 in a feedback path to a second input of theamplifier. Also connected to this second input is a resistor 122connected to circuit ground. Amplifier 110 and the feedback networkincluding resistors 120 and 122 comprise the gain control circuit 56 forsetting the output of the sample peak detector 40 equal to the output ofthe reference peak detector 42 when no gas is present in the sample chamber 10. The wiper arm of-the switch 118 connects to one input of anoperational amplifier 124 through resistors 126 and 128.

Operational amplifiers 112 and 124 and circuitry associated therewithcomprise the reference peak detector and sample peak detector,respectively. For the sample peak detector 40, the operational amplifier124 has an output coupled through a diode 130 to the collector electrodeof a transistor 132 and one side of a resistor 1 34. Transistor 132 hasan emitter electrode connected to ground and a base electrode connectedthrough a resistor 136 to a terminal 138 to which is applied a timingsignal for clearing both the peak detectors 40 and 42. Connected to thesecond terminal of the resistor 134 is a capacitor 140 and one input ofan operational amplifier 142. Capacitor 140 receives the series ofpulses generated by the detector 32 in response to light intensity inthe sample path and is charged to the level of the series of pulses andretains the charge until cleared by application of a signal to the baseelectrode of transistor 132 causing this transistor to turn on.

Connected to the output of the operational amplifier 142 is a feedbacknetwork including a variable resistor 144 and a resistor 146 connectedto ground. The interconnection of the resistors 144 and 146 ties to thesecond input of the operational amplifier 142. An output of theamplifier 142 generated at the junction of the resistor 144 is the peaksample path signal from the sample peak detector 40.

For the reference peak detector 42, the operational amplifier 112 has anoutput connected to a diode 152 in a feed-back path to the second inputof the amplifier. Also tied to the second input of the amplifier 1 12 isthe collector electrode of the transistor 154 and one terminal of aresistor 156. A second terminal of the resistor 156 connects to acapacitor 158 that receives and stores the series of pulses from thedetector 32 representing the intensity of the reference path energy.Transistor 154 has an emitter electrode connected to circuit ground anda base electrode connected to the terminal 138 through a resistor 160.Transistor 154 thus operates to complete the same function as transistor132, that is, to clear the reference peak detector 42 prior to receivingan updated series of pulses from the detector 32.

An interconnection of the capacitor 158 and the resistor 156 ties to oneinput of an operational amplifier 162 having an output coupled through afeedback network including a resistor 164 and a variable resistor 166 toa second input of the amplifier. The variable resistors 144 and 166 arean adjustment to balance the output of the peak detectors 40 and 42during a calibration cycle. Also connected to the second input of theamplifier 162 is a resistor 168 tied to ground. An output of theoperational amplifier 162 at the junction of the resistor 164 is theoutput of the reference peak detector 42.

As explained previously, each of the detectors 40 and 42 is gated ononly during the time necessary to receive the correct series of pulsesfrom the detector 32. For the peak detector 40, this gating circuitincludes a transistor 174 having a collector electrode connected toresistors 126 and 128 and an emitter electrode connected to ground. Thistransistor is controlled by a signal from the logic circuit 44 asapplied to the base electrode. The reference peak detector 42 includes atransistor 176 for gating the circuit to receive pulses from thedetector 32. Transistor 176 includes a collector electrode connected tothe resistors 114 and 116 and an emitter electrode connected to circuitground. Control of the transistor 176 is by a signal from a logiccircuit 44 as applied to the base electrode.

Voltages at the output of the operational amplifiers 142 and 162 areapplied to the logarithmic amplifier 58 that includes two amplifiers ingroups of two within the blocks 178 and operational amplifiers 180 and182. Connected to one input of an amplifier within the block 178 througha resistor 184 is the output of the reference peak detector 42. Thissignal is also applied to one input of an operational amplifier 180having a feedback path to a second input through a resistor 186 andresistor 187 to ground. An output of the amplifier 180 connects to anamplifier within the block 178 through a resistor 188. An output of thesample peak detector 40 is applied to an input of a second amplifierwithin the block 178 through a resistor 190 and this signal is alsoapplied to one input of the operational amplifier 182 having a secondinput connected to a feedback circuit including resistors 192 and 194.An output of the amplifier 182 connects to an input of the secondamplifier group of the block 178 through a resistor 196.

By the interconnection of the amplifiers 180 and 182 and the amplifierswithin the block 178, two outputs are generated. One output varies asthe logarithm of the amplitude of the sample path pulses and the secondvaries as the logarithm of the amplitude of the reference path pulses.These signals are coupled to separate inputs of an operational amplifier198 through resistors 200 and 202. Also connected to the resistor 202 isa resistor 204 connected to ground. A feedback path including a resistor206 and a resistor 208 is tied to the resistor 200. The resistor 208connects to the wiper arm of a variable resistor 210 having endterminals supplied from a minus and negative reference supply, to bedescribed.

The amplifier 198 and associated circuitry comprises the differentialamplifier 60 having an output connected to the scaling circuit 62 thatconsists of a twopole, four-position switch 212. Each of the upper fourpaths of the switch 212 include a variable resistor 214-217 and each ofthe lower paths include a fixed resistor 2l8222.

The upper arm of the switch 212 connects to one input of an operationalamplifier 224 having a second input connected to the lower wiper arm ofthe switch. A feedback path including a capacitor 226 and a resistor 228connects between the output of the amplifier 224 and one input thereof.Also connected to the output of the amplifier 224 is a variable resistor230 in series with the meter movement 68. A zero adjusting circuit forthe meter movement 68 includes a variable resistor 232 connected betweena minus reference voltage and ground.

As discussed previously, when an explosive concentration of hydrocarbongas exists in the sample chamber 10, the alarm circuit is triggered toturn on the audible alarm 72. The alarm circuit 70 includes a variableresistor 234 connected to the output of the operational amplifier 162and to a minus reference supply at terminal 236. The wiper arm of thevariable resistor 234 ties to one input of a comparator 238 having asecond input grounded through a line 240. An output of the comparator238 drives a switching circuit including a transistor 242 having a baseresistor 244 connected to the amplifier 238. A resistor 246interconnects between the emitter electrode of the transistor 242 andthe base electrode thereof. Connected to the collector electrode of thetransistor 242 is the alarm 72 which may comprise a horn, bell or otheraudible or visual indicator.

To provide the reference voltages to the various subcircuits throughoutthe circuit of FIG. 4, a DC voltage as supplied by a battery provides anegative voltage to a terminal 248 and a positive voltage to a terminal250. The circuit for supplying the negative reference voltage connectsto the terminal 248 and includes a Zener diode 252, a resistor 254, acapacitor 256 and a Zener diode 258. The negative reference voltage isgenerated at the junction of the resistor 254, the capacitor 256 and theanode electrode of the Zener diode 258. The circuit for generating thepositive reference voltage includes a resistor 260 connected to theterminal 250, a capacitor 262 and a Zener diode 264 connected to thesecond terminal of the resistor 260. The positive reference voltageappears at the interconnection of the resistor 260 and the capacitor 262and Zener diode 264.

Referring to FIG. 5, timing signals for gating the laser 16 and the peakdetectors 40 and 42 are generated in response to a signal from thephotodetector 46 which typically may be a light responsive transistorhaving a collector electrode connected to a resistor 268 and the outputof a regulated supply at terminal 266. An emitter electrode of the diode46 ties to the base of a transistor 270 having a grounded emitter and acollector electrode connected to the resistor 268.

A signal at the collector of the transistor 270 is applied through adiode 272 to one input of a NAND gate 274. A second input to the NANDgate 274 is coupled directly to the collector electrode of thetransistor 270. Also connected to the second input of the NAND gate 274is a capacitor 276. Connected to the output of the gate 274 is a NANDgate 280 having an output connected to a resistor 284 for controllingthe sequential operation of the gates 274 and 280.

Timing signals generated at the output of the gate 280 are applied toone input of a binary counter 286 interconnected to provide varioustiming signals to the remainder of the logic circuit 44. One series oftiming pulses from the binary counter 286 is applied to the input of asingle counter 288 and a NAND gate 290. An output of the single shotgate 288 also connects to an input of the NAND gate 290 and a secondseries of pulses from the register 286 connects to the NAND gate 290.Logic level pulses from the gate 290 are applied to a NAND gate 292 andin addition represent the triggering pulses for gating on the laser 16as applied to the terminal 54 through a resistor 294.

The NAND gate 292 serves to invert the output of a NAND gate 290 andthis signal is applied to one input of a NAND gate 296 having a secondinput from the counter 286. Signals generated at the output of the NANDgate 296 are coupled to one input of a NOR gate 298, one input of a NORgate 300 and an input of a NAND gate 302 through a capacitor 304. Alsocoupled to the input of the NAND gate 302 is a circuit includingresistors 306 and 308 and a diode 310. An output from the NAND gate 302is a series of logic timing pulses coupled to the base electrodes of thetransistors 132 and 154 for clearing the peak detector capacitors 140and 158,

A second input to the NOR gate 298 is the output of the NAND gate 280.The NOR gate 298 provides one input to a NAND gate 312 having a secondinput from the counter 286. An output of the NAND gate 312 is a seriesof timing pulses applied to the base electrode of transistor 174 throughresistor 314. The NOR gate 300 has a second input from the output of theNAND gate 274 and provides a series of pulses to a NOR gate 316 thatinverts the output of the gate 300. An output of the NOR gate 316 is aseries of timing pulses for controlling the transistor 176 through aresistor 318.

Thus, the logic circuit 44 as detailed in FIG. 5 provides four series oftiming pulses controlling the operation of the system of FIG. 2,specifically gating the peak detectors and 42 and triggering the laser54 at the proper times.

While only one embodiment of the invention, together with modificationsthereof, has been described in detail herein and shown in theaccompanying drawings, it will be evident that various furthermodifications are possible without departing from the scope of theinvention.

What is claimed is:

l. A hydrocarbon gas detector having a monochromatic light sourceemitting radiation in the region absorbent of hydrocarbon molecules,said beam split into a sample path beam and a reference path beam bothdirected through a sample chamber along separate paths to a detectormeans having an output related to the energy in the sample path andreference path, the improvement comprising:

a sample channel circuit responsive to the output signal of the detectormeans related to the sample path, energy for generating a sample channeloutput,

a reference channel circuit responsive to the output signal of thedetector means related to the reference path energy for generating areference channel output,

a logarithmic amplifier connected to the output of the sample channelcircuit and the reference chan- 10 nel circuit and generating twooutputs, one varying as the logarithm of the sample channel output andthe second varying as the logarithm of the reference channel output,

a differential amplifier connected to the outputs of said logarithmicamplifier and having an output varying as the ratio of the samplechannel output over the reference channel output, and

output circuit means responsive to the output of said differentialamplifier to generator a measure of the concentration of hydrocarbon gasin the sample chamber.

2. A hydrocarbon gas detector as set forth in claim 1 including an alarmresponsive to a predetermined level of output signal from said referencechannel circuit to indicate a concentration of hydrocarbon gas in thetest chamber above an established level.

3. A hydrocarbon gas detector as set forth in claim 1 including a gaincontrol adjustment connected to said sample channel circuit to adjustthe output thereof to be substantially equal to the output of thereference channel circuit with a hydrocarbon free gas sample in the testchamber.

4. A hydrocarbon gas detector as set forth in claim 1 including circuitmeans for timing the response of the sample channel circuit and thereference channel circuit to the detector output only when therespective path energy is applied to said detector.

5. A hydrocarbon gas detector as set forth in claim 4 wherein saidtiming means connects said sample channel circuit and said referencechannel circuit to the detector output at preselected time intervals.

6. A hydrocarbon gas detector as set forth in claim 5 wherein saidsample channel circuit and said reference channel circuit each include agated peak detector responsive to the output of the detector meansduring the preselected time intervals and storing the detector outputuntil a subsequent gating time interval.

7. A hydrocarbon gas detector having a monochromatic light sourceemitting radiation in a region absorbent of hydrocarbon molecules,comprising in combination:

means for splitting the radiation into a sample path beam and areference path beam both through a sample chamber containing a gassample along separate paths,

radiant energy detector means responsive to energy in the sample pathand the reference path and generating an output signal related to each,

a sample channel circuit responsive to the output signal of saiddetector means related to the sample path energy for generating a samplechannel output,

a reference channel circuit responsive to the output signal of saiddetector means related to the reference path energy for generating areference channel output,

means for establishing a timing interval for connecting said samplechannel circuit to said detector means to respond only to the samplepath energy and for sequentially connecting the reference channelcircuit to said detector means to respond only to the reference pathenergy, and

output circuit means responsive to an output from said sample channelcircuit and said reference channel circuit to generate a measure of thehydrocarbon gas in the sample chamber.

8. A hydrocarbon gas detector as set forth in claim 7 wherein saidoutput circuit means includes a differential amplifier responsive to theoutput of the sample channel circuit and the reference channel circuitand providing an output related thereto.

9. A hydrocarbon gas detector as set forth in claim 7 wherein saidoutput circuit means includes a logarithmic amplifier connected to theoutput of the sample channel circuit and the reference channel circuitand generating two outputs, one varying as the logarithm of the samplechannel output and the second varying as the logarithm of the referencechannel output, and

a differential amplifier connected to the outputs of said logarithmicamplifier and having an output varying as the ratio of the samplechannel output over the reference channel output.

10. A hydrocarbon gas detector as set forth in claim 9 wherein saidoutput circuit means further includes an indicating meter responsive tothe output of said differential amplifier.

11. A hydrocarbon gas detector as set forth in claim 7 including analarm responsive to a predetermined level of output signal from saidreference channel circuit to indicate a concentration of hydrocarbon gasin the sample chamber above an established level.

12. A hydrocarbon gas detector as set forth in claim 7 including a gaincontrol adjustment connected to said sample channel circuit to adjustthe output thereof to be substantially equal to the output of thereference channel circuit with a hydrocarbon free gas sample in thesample chamber.

13. A hydrocarbon gas detector as set forth in claim 7 wherein saidmeans for establishing a timing interval connects said sample channelcircuit and said reference channel circuit to said radiant energydetector means at preselected time intervals.

14. A hydrocarbon gas detector as set forth in claim 13 wherein saidsample channel circuit and said reference channel circuit each include agated peak voltage detector responsive to the output of said energydetector means during the selected predetermined time intervals andstoring the detector output until a subsequent gating time interval.

15. A hydrocarbon gas detector, comprising in combination:

a monochromatic light source emitting radiation in the region absorbentof hydrocarbon molecules, means for splitting the radiation into asample path beam and a reference path beam,

a sample chamber containing a gas sample,

a radiant energy detector,

means for directing the sample beam through said sample chamber in afirst direction to said detector means,

means for directing the reference beam through said test chamber in asecond separate direction to said detector means,

a sample channel circuit responsive to the output signal of saiddetector related to the sample path energy for generating a samplechannel output,

a reference channel circuit responsive to the output signal of saiddetector related to the reference path energy for generating a referencechannel output,

means for establishing a timing interval for connecting said samplechannel circuit to said detector to respond only to the sample channelenergy and for connecting the reference path circuit to said detector torespond only to the reference path energy, and

output circuit means responsive to an output from said sample channelcircuit and said reference channel circuit to generate a measure ofhydrocarbon gas in the sample chamber.

16. A hydrocarbon gas detector as set forth in claim 15 wherein saidmeans for establishing a timing interval includes means for gating themonochromatic light source on for a preselected time interval.

17. A hydrocarbon gas detector as set forth in claim 16 wherein saidmeans for splitting the radiation beam into a sample path and areference path includes triggering means for actuating the means forestablishing a timing interval.

18. A hydrocarbon gas detector as set forth in claim 17 wherein saidmeans for establishing a timing interval includes synchronizing meansfor connecting said sample channel circuit and said reference channelcircuit to the detector output at preselected time intervals during anon cycle of said monochromatic light source.

19. A hydrocarbon as detector as set forth in claim 15 wherein saidoutput circuit means includes a logarithmic amplifier connected to theoutput of the sample channel circuit and the reference channel circuitand generating two outputs, one varying as the logarithm of the samplechannel output and the second varying as the logarithm of the referencechannel output, and

a differential amplifier connected to the outputs of said logarithmicamplifier and having an output varying as the ratio of the samplechannel output over the reference channel output.

20. A hydrocarbon gas detector as set forth in claim 1.9 including analarm responsive to a predetermined level of output signal from saidreference channel circuit to indicate a concentration of hydrocarbon gasin the sample chamber above an established level.

21.'A hydrocarbon gas detector as set forth in claim 20 including a gaincontrol adjustment connected to said sample channel circuit to adjustthe output thereof to be substantially equal to the output of thereference channel circuit with a hydrocarbon free sample in the samplechamber.

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1. A hydrocarbon gas detector having a monochromatic light sourceemitting radiation in the region absorbent of hydrocarbon molecules,said beam split into a sample path beam and a reference path beam bothdirected through a sample chamber along separate paths to a detectormeans having an output related to the energy in the sample path andreference path, the improvement comprising: a sample channel circuitresponsive to the output signal of the detector means related to thesample path, energy for generating a sample channel output, a referencechannel circuit responsive to the output signal of the detector meansrelated to the reference path energy for generating a reference channeloutput, a logarithmic amplifier connected to the output of the samplechannel circuit and the reference channel circuit and generating twooutputs, one varying as the logarithm of the sample channel output andthe second varying as the logarithm of the reference channel output, adifferential amplifier connected to the outputs of said logarithmicamplifier and having an output varying as the ratio of the samplechannel output over the reference channel output, and output circuitmeans responsive to the output of said differential amplifier togenerator a measure of the concentration of hydrocarbon gas in thesample chamber.
 2. A hydrocarbon gas detector as set forth in claim 1including an alarm responsive to a predetermined level of output signalfrom said reference channel circuit to indicate a concentration ofhydrocarbon gas in the test chamber above an established level.
 3. Ahydrocarbon gas detector as set forth in claim 1 including a gaincontrol adjustment connected to said sample channel circuit to adjustthe output thereof to be substantially equal to the output of thereference channel circuit with a hydrocarbon free gas sample in the testchamber.
 4. A hydrocarbon gas detector as set forth in claim 1 includingcircuit means for timing the response of the sample channel circuit andthe reference channel circuit to the detector output only when therespective path energy is applied to said detector.
 5. A hydrocarbon gasdetector as set forth in claim 4 wherein said timing means connects saidsample channel circuit and said reference channel circuit to thedetector output at preselected time intervals.
 6. A hydrocarbon gasdetector as set forth in claim 5 wherein said sample channel circuit andsaid reference channel circuit each include a gated peak detectorresponsive to the output of the detector means during the preselectedtime intervals and storing the detector output until a subsequent gatingtime interval.
 7. A hydrocarbon gas detector having a monochromaticlight source emitting radiation in a region absorbent of hydrocarbonmolecules, comprising in combination: means for splitting the radiationinto a sample path beam and a reference path beam both through a samplechamber containing a gas sample along separate paths, radiant energydetector means responsive to energy in the sample path and the referencepath and generating an output signal related to each, a sample channelcircuit responsive to the output signal of said detector means relatedto the sample path energy for generating a sample channel output, areference channel circuit responsive to the output signal of saiddetector means related to the reference path energy for generating areference channel output, means for establishing a timing interval forconnecting said sample channel circuit to said detector means to respondonly to the sample path energy and for sequentially connecting thereference channel circuit to said detector means to respond only to thereference path energy, and output circuit means responsive to an outputfrom said sample channel circuit and said reference channel circuit togenerate a measure of the hydrocarbon gAs in the sample chamber.
 8. Ahydrocarbon gas detector as set forth in claim 7 wherein said outputcircuit means includes a differential amplifier responsive to the outputof the sample channel circuit and the reference channel circuit andproviding an output related thereto.
 9. A hydrocarbon gas detector asset forth in claim 7 wherein said output circuit means includes alogarithmic amplifier connected to the output of the sample channelcircuit and the reference channel circuit and generating two outputs,one varying as the logarithm of the sample channel output and the secondvarying as the logarithm of the reference channel output, and adifferential amplifier connected to the outputs of said logarithmicamplifier and having an output varying as the ratio of the samplechannel output over the reference channel output.
 10. A hydrocarbon gasdetector as set forth in claim 9 wherein said output circuit meansfurther includes an indicating meter responsive to the output of saiddifferential amplifier.
 11. A hydrocarbon gas detector as set forth inclaim 7 including an alarm responsive to a predetermined level of outputsignal from said reference channel circuit to indicate a concentrationof hydrocarbon gas in the sample chamber above an established level. 12.A hydrocarbon gas detector as set forth in claim 7 including a gaincontrol adjustment connected to said sample channel circuit to adjustthe output thereof to be substantially equal to the output of thereference channel circuit with a hydrocarbon free gas sample in thesample chamber.
 13. A hydrocarbon gas detector as set forth in claim 7wherein said means for establishing a timing interval connects saidsample channel circuit and said reference channel circuit to saidradiant energy detector means at preselected time intervals.
 14. Ahydrocarbon gas detector as set forth in claim 13 wherein said samplechannel circuit and said reference channel circuit each include a gatedpeak voltage detector responsive to the output of said energy detectormeans during the selected predetermined time intervals and storing thedetector output until a subsequent gating time interval.
 15. Ahydrocarbon gas detector, comprising in combination: a monochromaticlight source emitting radiation in the region absorbent of hydrocarbonmolecules, means for splitting the radiation into a sample path beam anda reference path beam, a sample chamber containing a gas sample, aradiant energy detector, means for directing the sample beam throughsaid sample chamber in a first direction to said detector means, meansfor directing the reference beam through said test chamber in a secondseparate direction to said detector means, a sample channel circuitresponsive to the output signal of said detector related to the samplepath energy for generating a sample channel output, a reference channelcircuit responsive to the output signal of said detector related to thereference path energy for generating a reference channel output, meansfor establishing a timing interval for connecting said sample channelcircuit to said detector to respond only to the sample channel energyand for connecting the reference path circuit to said detector torespond only to the reference path energy, and output circuit meansresponsive to an output from said sample channel circuit and saidreference channel circuit to generate a measure of hydrocarbon gas inthe sample chamber.
 16. A hydrocarbon gas detector as set forth in claim15 wherein said means for establishing a timing interval includes meansfor gating the monochromatic light source on for a preselected timeinterval.
 17. A hydrocarbon gas detector as set forth in claim 16wherein said means for splitting the radiation beam into a sample pathand a reference path includes triggering means for actuating the meansfor establishing a timing interval.
 18. A hydrocarbon gas detector asset forth in claim 17 wherein said means for establishing a timinginterval includes synchronizing means for connecting said sample channelcircuit and said reference channel circuit to the detector output atpreselected time intervals during an ''''on'''' cycle of saidmonochromatic light source.
 19. A hydrocarbon as detector as set forthin claim 15 wherein said output circuit means includes a logarithmicamplifier connected to the output of the sample channel circuit and thereference channel circuit and generating two outputs, one varying as thelogarithm of the sample channel output and the second varying as thelogarithm of the reference channel output, and a differential amplifierconnected to the outputs of said logarithmic amplifier and having anoutput varying as the ratio of the sample channel output over thereference channel output.
 20. A hydrocarbon gas detector as set forth inclaim 19 including an alarm responsive to a predetermined level ofoutput signal from said reference channel circuit to indicate aconcentration of hydrocarbon gas in the sample chamber above anestablished level.
 21. A hydrocarbon gas detector as set forth in claim20 including a gain control adjustment connected to said sample channelcircuit to adjust the output thereof to be substantially equal to theoutput of the reference channel circuit with a hydrocarbon free samplein the sample chamber.